Effect of cohesion on the gravity-driven evacuation of metal powder through Triply-Periodic Minimal Surface structures

TL;DR

This study tackles the de-powdering challenge in metal-powder additive manufacturing by using Discrete Element Method (DEM) simulations to model gravity-driven evacuation of cohesive powders from Triply Periodic Minimal Surface (TPMS) unit cells. It develops a DEM framework with Hertzian contact, Coulomb friction, and SJKR cohesion, embedding Schwarz-P and Gyroid (and Diamond, I-WP) shells within a cubic particle packing and comparing discharge behavior across cohesion levels via the cohesion energy density $k_c$. The key findings are that Schwarz-P and Gyroid geometries enable the most efficient evacuation and are comparatively robust to cohesion, while other geometries are more sensitive to cohesion, with detailed kinematic and force-chain analyses revealing transient arching and load transmission as the main flow-limiting mechanisms. The work provides geometry-aware design guidance for de-powdering TPMS-based AM components and points to future extensions including non-spherical powders, electrostatic effects, and CFD–DEM coupling to capture air-assisted evacuation.

Abstract

Evacuating the powder trapped inside the complex cavities of Triply Periodic Minimal Surface (TPMS) structures remains a major challenge in metal-powder-based additive manufacturing. The Discrete Element Method offers valuable insights into this evacuation process, enabling the design of effective de-powdering strategies. In this study, we simulate gravity-driven evacuation of trapped powders from inside unit cells of various TPMS structures. We systematically investigate the role of cohesive energy density in shaping the discharge profile. Overall, we conclude that the Schwarz-P and Gyroid topologies enable the most efficient powder evacuation, remaining resilient to cohesion-induced flow hindrance. Furthermore, for the two unit cells, we analyse detailed kinematics and interpret the results in relation to particle overlaps and contact force distributions.

Figures (11)

• Figure 1: $2\times2\times2$ array-structures built from TPMS unit cells commonly employed in Additive Manufacturing (AM). Panel (a) is based on a Schwarz-P unit cell, (b) on a Gyroid, and (c) on a Diamond unit cell. The two disjoint regions in each of the structures are shown in orange and cyan.
• Figure 2: (a) Schematic of the Powder Bed Fusion-based AM of a bunker, and (b) the flow regimes encountered during the evacuation of the unused trapped powder. (b)[i] shows mass flow, where the entire granular medium is in motion, (b)[ii] and b[iii] show funnel flow, where there's a slow-moving/ stagnated region of material close to the bunker with a faster moving central core, with the passive region being localised at the outlet in the former, and spread over the entire system in the latter.
• Figure 3: Cohesion energy density and the corresponding equilibrium contact force for a given Hertzian spring. (a) shows the equilibrium overlaps and contact forces for two identical spheres modelled as Hertzian springs of stiffness 2.11 $MPa$ at various cohesive energy densities, $k_c$, varying from 0 to 45 $kJ/m^3$, as red dots. (b) shows the variation of the normalised equilibrium forces as a function of $k_c$.
• Figure 4: Various Triply Periodic Minimal Surface (TPMS) unit cells, with their names shown in the respective sub-cations. The colours cyan and orange demarcate the disjoint regions of space partitioned by the surfaces.
• Figure 5: Variation of particle retention inside the unit cell with normalised time for (a) Schwarz-P and (b) Gyroid structures at different cohesive energy densities. $k_c$ varies from 0 to 45 $kJ/m^3$. The common legend is shown in (b).